Sometimes we do things that don't make much business sense. Sometimes business exists despite this. Or, perhaps, it exists in part because there are those who dare to buck what the pros say.
Picture book writers often operate, and even flourish, using practices that seem at odds with the market, such as ignoring trends and working without an agent. Why? Passion is the most obvious reason. But there are other practical reasons for not seeking the services of an agent. First, so few take on new PBs writers. Second, most that do, won't rep the whole body of a PB writer's work, opting to push books that they have a strong connection with themselves. Third, I've heard many writers complain that agents try to drive them into writing only on certain topics. Some PB writers don't operate well under such constraints - or won't.
Those are only a few of the reasons.
To get a sense for the firestorm Caren Johnson Agency's Elana Roth's Thursday post about the relationship between agents and PB writers has caused (which I responded to yesterday here), check out this thread at Verla Kay's Children's Writers and Illustrators Discussion Boards.